Tungsten(VI) phenylimido alkylidene complexes containing a monoanionic O,N-chelating ligand and their isolated precursor complexes: X-ray structures of W(CH2SiMe3)3(=NPh)[OCPh2(2-py)] and W(=CHSiMe3)(CH2SiMe3)(=NPh)[OCPh 2(2-py)]

Article abstract of DOI:10.1021/om00016a051

The lithium salts of selected phenoxides, with one or two potentially chelating o-CH₂NMe₂ substituents, 8-quinolinolate (8-quin), ortho-chelating arenethiolate ligands, and α-substituted 2-pyridylmethoxides, were reacted with W(CH₂SiMe₃)₃Cl(=NPh) (1) and W(CH₂SiMe₃)₂Cl(=NPh)(OCMe₃) (2), respectively. It appears that steric properties of the monoanionic O,N-and S,N-chelating ligands are very important for inducing an intramolecular H_α-abstraction reaction; i.e., the phenoxide OC₆H₄CH₂NMe₂-2 (a) gave rise to the formation of alkylidene precursor W(CH₂SiMe₃)₃(=NPh)(OC₆H ₄CH₂NMe₂-2) (3), in which the alkyl groups are still intact, whereas the pincer phenolate OC₆H₂(CH₂NMe₂)₂-2,6-Me-4 (b), with two o-CH₂NMe₂ substituents, directly led to the formation of alkylidene complex W(=CHSiMe₃)(CH₂SiMe₃)(=NPh)(OC ₆H₂(CH₂NMe₂)₂-2,6-Me-4) (4′). The reaction of 1 with Na-8-quin instantaneously gave W(=CHSiMe₃)(CH₂SiMe₃)(=NPh)(8-quin) (5′). Also, electronic properties play an important role in the formation of alkylidene complexes; i.e., the complexes derived from 2, containing a π-electron-donating tert-butoxide group, are thermally more stable toward intramolecular H_α abstraction than those derived from 1. With the α-substituted 2-pyridylmethoxide ligands OR¹R²(2-py), in which the R substituents can be varied easily, we were able to prepare both the alkylidene precursor and the subsequent product of an intramolecular H_α-abstraction reaction, the tungsten(VI) alkylidene complex. The solid-state structure (X-ray) of the trialkyltungsten(VI) precursor W(CH₂SiMe₃)₃(=NPh)[OCPh₂(2-py)] (9) as well as the resulting alkylidene complex W(=CHSiMe₃)(CH₂SiMe₃)(=NPh)[OCPh ₂(2-py)] (9′) have been determined. Crystals of 9 are monoclinic, space group P2₁/c, with unit-cell dimensions a = 10.974(1) A?, b = 18.531(3) A?, and c = 20.307(2) A?, β = 106.47(1)°, Z = 4, final R = 0.063, and R_w = 0.053 for 2644 reflections with I > 2.5σ(I) and 415 parameters. Crystals of 9′ are triclinic, space group P1, with unit-cell dimensions a = 9.4359(5) A?, b = 10.8887(7) A?, and c = 16.611(1) A?, α = 81.60(1)°, β = 88.37(1)°, γ = 79.19(1)°, Z = 2, final R = 0.042, and R_w = 0.026 for 5879 reflections with I > 2.5σ(I) and 348 parameters. The two molecular structures clearly show the advantage of a d⁰ metal center in forming an alkylidene unit out of two alkyl groups. The chemical shift of H⁶, the hydrogen atom bonded at the ortho position with respect to the pyridyl nitrogen, as a probe shows that the pyridyl group in alkylidene complexes 9′ and W(=CHSiMe₃)(CH₂SiMe₃)(=NPh)[OCH(CMe ₃)(2-py)] (10′) can coordinate intramolecularly in the syn rotamer, whereas in the anti rotamer this will lead to considerable steric hindrance of the SiMe₃ group of the alkylidene function with the vicinal substituents in the chelating ring. The reactivity of the present tungsten(VI) alkylidene complexes 5′, 9′, and 10′ toward linear olefins is very low. However, aldehydes react with these complexes in Wittig-type reactions to yield predominantly the corresponding trans-olefin. Norbornene can be polymerized using these alkylidene complexes as a catalyst in a ring-opening metathesis reaction at 70°C to give polymeric cyclopentenes with ≥90% cis vinylene bonds.